Strings for tennis, squash and badminton rackets are required to have specific characteristics of resistance to pulling and to elongation under a brief constraint or under repeated constraints; in these latter conditions, they should rapidly and totally take up their initial length again; finally, they should have good properties of resistance to different condition of use, notably abrasion resistance, resistance to creasing or kinking, resistance to various atmospheric factors as well as to the various constraints to which they are subjected during their fitting to rackets etc.
Strings of animal gut have been used for a very long time on high quality tennis and other rackets and have proved wholly acceptable from the point of view of power, feel and playability, but unfortunately show a poor resistance to moisture which shortens their playing life when humid conditions prevail. The elastic return characteristics (rapid and total return to the initial length after a brief constraint or repeated constraints) of natural gut are, however, excellent.
Apart from nylon monofilament which has been used extensively since 1944, strings made from other thermoplastic polymeric materials are also known from the patent literature:
U.S. Pat. No. 4,300,343 is concerned with a synthetic gut prepared by collectively twisting a plurality of monofilaments of a thermoplastic resin at a temperature higher than the softening point of said resin, thereby producing a gut wherein the monofilaments in the central portion of the gut adhere to one another such that the independent shape of each monofilament cannot be distinguished and wherein the monofilaments at the periphery of the gut adhere to one another while maintaining their independent shape. The monofilaments in the gut are made from a fluorocarbon resin, particularly a vinylidene fluoride resin, a polyamide resin or a polyester resin.
British Pat. No. 1 578 599 is concerned with a racket string consisting of from 2 to 4 monofilaments of an oriented, synthetic thermoplastic polymer, more particularly nylon 66 or nylon 6, with each monofilament having a denier of 2,000 to 8,000 and at least two flattened sides, two of which are opposed to one another, throughout its length, said monofilaments having substantially no individual twist and being ply-twisted and bonded together throughout the length of the string with each said monofilament being bonded along a flattened side to at least one other of said monofilaments.
British Pat. No. 1 569 530 describes a sports racket string comprising a substantially circular cross-section core of one or more synthetic resin monofilaments and an outer helically wound wrapping of synthetic resin monofilaments, which may be the same as or different from the synthetic resin material of the core, the wrapping being formed from monofilaments of at least two different diameters arranged so that along the length of the string there are alternately portions of surface comprising smaller diameter monofilaments and raised portions of surface comprising at least one larger diameter monofilament. The monofilaments used may be of a polyester, such as polyethylene terephthalate, or a nylon.
U.S. Pat. No. 4,275,117 is concerned with a racket string resulting from the integration under heat of a combination of elongated strands of a first and a second thermoplastic material, said first thermoplastic material having a substantially higher melting point than said second thermoplastic material, said string having been integrated by the application of heat sufficient to melt said second material but not said first material, said string, prior to integration, having a compressed core consisting at least in part of said second material, and a braided sheath over said core comprising strands of both said first and second materials. Nylon 66 having a melting point of approximately 480.degree. F. is given as an example of the higher melting point thermoplastic material and a nylon ter-polymer having a melting point of approximately 310.degree. F. is given as an example of the lower melting point thermoplastic material.
U.S. Pat. No. 4,328,055 is concerned with a method of preparing a synthetic gut comprising melt spinning a thermoplastic resin, more particularly a polyvinylidene fluoride resin, polyamide resin or a polyester resin, into a plurality of monofilaments, collectively twisting the plurality of monofilaments while the monofilaments are maintained at a temperature higher than the softening point of the resin thereby obtaining a gut having a structure consisting of a melt adhered nuclear part and a spiral peripheral part of the melt-adhered monofilaments.
U.S. Pat. No. 4,391,088 is concerned with a sports racket string consisting of a natural gut core covered with a filamentary aramid and impregnated with a coating of a water-resistant, vapour-impermeable flexible adhesive polymeric resin which adheres the filamentary aramid to the gut core.
U.S. Pat. No. 4,084,399 is concerned with a synthetic gut made from carbon fibres optionally combined with organic and/or inorganic fibres.
British Pat. No. 1 587 931 is concerned with a twisted bundle of synthetic multifilament yarns which are adhered together by a thermosettable adhesive. The yarns may be of nylon, polyester or an aromatic polyamide.